The present invention pertains to the signal processing art and in particular, to a multipole resonator adapted for application as a bandpass filter.
Multipole resonators for use in bandpass filter applications are well known, especially in the communication art. There, for example, radio frequency receivers employ intermediate frequency filters which selectively pass signals within a given bandwidth, providing significant attenuation to signals outside the given band. The characteristics of an ideal bandpass filter are a relatively linear passband response and infinite rejection to signals outside the passband. This ideal is only approximated, with most bandpass filters having some ripple in the bassband, and displaying less than ideal attenuation characteristics outside of the band.
Piezoelectric elements, particularly quartz crystals, find extended use in multipole bandpass filters. FIG. 1 illustrates a typical dual coupled four pole monolithic crystal filter in circuit configuration with a signal generator 12, having a source impedance 14, and a filter load impedance 16. The filter includes two dual coupled monolithic crystals filters 18 and 20. Since the configuration and electrical characteristics of the second two pole crystal element 20 are identical to that of the first element 18, only the element 18 will be described in detail. The two pole element 18 is comprised of a central piezoelectric element 22, preferably quartz, which has deposited thereon predeterminedly positioned electrodes 24, 26 and the ground electrode 28. Input signals are applied at the input pair of electrodes, 24 and 28 and are thereafter acoustically coupled through the quartz element 22 whereby they may be extracted from the output electrodes 26 and 28. A tank circuit 30 is shown in shunt with the input electrodes 24, 28, for tuning out the input capacitance Co of the input reasonator 24, 28. A similar tank circuit 32 is shown coupled across the output of the second crystal element 20, for providing the same tuning purpose as the first tank circuit 30. A capacitor 36 is in shunt between the two filter elements 18, 20, supplying an appropriate coupling between elements 18 and 20.
FIG. 2 illustrates by a solid line 40 the frequency response plot of a single dual coupled resonator, such as resonator 18. As FIG. 2 illustrates, the single resonator has a relatively flat passband characteristic and steep side skirts, but displays spurs outside of the passband which significantly degrade filter performance. When the two, two pole crystal filters 18 and 20 are cascaded, thus forming a four pole filter, the response plot is as shown by the dotted line 42. By cascading the two filters, the results are essentially the algebraic summation of two response plots from a single two pole monolithic crystal filter. Thus, the effect is essentially to maintain the passband response but steepen the side skirts. Also, cascading more greatly attenuates the spurs relative to the passband. Notice, however, that due to the algebraic summation of the single response as shown at 40, the spurs in the cascaded response plot at 42 are essentially twice their original value. Thus, cascading two stages does not optimally reduce spur size.
Hence, the prior art has felt a need for an effective, yet inexpensive bandpass filter which exhibits a relatively flat passband, has steep side skirts, and provides optimum spur suppression.